The present invention relates to an arrangement in connection with a discharge lamp, the arrangement comprising an electronic ballast for igniting and burning the discharge lamp, and a voltage control device arranged to modify supply voltage of the ballast and to include a power control signal in the supply voltage.
The brightness of incandescent lamps can be controlled, i.e. dimmed, simply by adjusting the working voltage supplied thereto. In alternating current arrangements, the effective value of the voltage is controlled using a method called clipping control. In the clipping control, a thyristor or another semiconductor component is made conductive only for part of the duration of a half cycle of the voltage. In practice, then, part of a sine wave of the mains voltage is cut off. The adjustment is thus carried out by controlling the ignition angle of the semiconductor component in the half cycle.
Direct current arrangements employ pulsed, i.e. pulse-width-modulated (PWM), direct current in order to keep the efficiency of the control device good. A mean value of the pulsed direct current is then formed according to a pulse ratio, i.e. a 50% pulse ratio corresponds to about a 50% voltage value. When pulse frequency is sufficiently high (e.g. 50 Hz), the human eye perceives light as unflickering. This is due to the slowness of the eye and to the thermal mass of an incandescent filament, which makes the temperature of the incandescent filament slow to change. In control arrangements for incandescent lamps, the control may be located separately from the incandescent lamp; most typically, it is installed in connection with a light switch.
In a fluorescent light arrangement or in other discharge lamps, controlling is far more complex since a fluorescent tube necessitates separate power feeds for the tube voltage and the filament voltages when a modern electronic ballast is used. Therefore, the fluorescent tube requires a separate ballast located in connection with the fluorescent tube. The ballast provides the cathodes, i.e. the filaments, of the fluorescent tube with a voltage of their own and the tube with a voltage of its own. When light is adjusted, the tube voltage or tube current and the filament voltage are controlled separately from each other. For operational reasons, the control cannot be located in connection with a lamp since lamps are often located in places that are difficult to reach, such as a ceiling. Thus, the control has to be located in a place from which lights are usually controlled. In such a case, in addition to current feed wires, the controlling also necessitates a separate control wire or a twin wire to the ballast. The need for several wires is impractical and it makes a control solution difficult to install as a replacement for a lamp operating on a normal principle.
Adjustable lighting implemented using fluorescent tubes would be ideal for several different places as far as both energy economy and user-friendliness are concerned. The color-rendering properties of the light produced by the fluorescent tubes are unparalleled over lighting implemented using common incandescent lamps. Typically, fluorescent tubes can be used for adjustable lighting e.g. in auditoria, assembly rooms, theatres and public transportation vehicles. Furthermore, adjustable lighting can be used for making residential buildings much more comfortable, practical and adjustable.
U.S. Pat. No. 5,107,184 presents a system for dimming a fluorescent lamp. In this publication, dimming information is coded within the supply voltage waveform by using power flow interruption coding. In this coding, the information is coded by interrupting the current flow to the ballast in a predetermined sequence. The system of the publication can only be used in connection with AC power.
An object of the present invention is to provide an arrangement which avoids the above-mentioned drawbacks and which enables the level of light of a fluorescent tube to be controlled using a simple apparatus without separate wires for the controlling. This object is achieved by an arrangement of the invention, which is characterized in that the voltage control device is arranged to modify the base-frequency pulse shape of the supply voltage of the ballast to include power control information in the supply voltage, and that the arrangement further comprises a power filter and a control filter for separating the power control signal from the supply voltage, the ballast being responsive to the power control signal in order to control the level of light of a fluorescent tube.
The arrangement of the invention is based on the idea that a voltage control device, such as a control implementing clipping control, is used for modifying the supply voltage in a manner similar to that in connection with an incandescent lamp. From the pulsed voltage, a power control signal is then separated using a separate control filter, and on the basis of the signal, a ballast is used for conveying the desired current to the fluorescent tube in order to burn the fluorescent tube at a desired level of brightness. The arrangement enables the fluorescent tube to be controlled without an additional control wire or twin wire, allowing a simple control previously possible only in connection with incandescent lamps to be used for adjusting the brightness.